The present invention relates to an engine exhaust purification device provided with a catalyst.
JP-A-H9-228873 published by the Japanese Patent Office in 1997 discloses a technique wherein an oxygen amount stored in a three-way catalyst (hereafter, xe2x80x9coxygen storage amountxe2x80x9d) is estimated based on an engine intake air amount and an air fuel ratio of an exhaust flowing into the catalyst, and engine air-fuel ratio control is performed so that the oxygen storage amount of the catalyst is constant.
To maintain the NOx (nitrogen oxides), CO and HC (hydrocarbon) conversion efficiency of the three-way catalyst at a maximum, the catalyst atmosphere must be maintained at the stoichiometric air-fuel ratio. If the oxygen storage amount of the catalyst is maintained constant, oxygen in the exhaust is stored in the catalyst even if the air-fuel ratio of the exhaust flowing into the catalyst temporarily becomes lean, and conversely, oxygen stored in the catalyst is released even if the air-fuel ratio of the exhaust flowing into the catalyst temporarily becomes rich, so the catalyst atmosphere can be maintained at the stoichiometric air-fuel ratio.
Therefore, in an exhaust purification device performing this type of control, it is required to calculate the oxygen storage amount precisely to maintain the conversion efficiency of the catalyst at a high level, and various methods of computing the oxygen storage amount have been proposed.
However, in the prior art, as the oxygen storage amount was computed without considering the variation of catalyst characteristic in the engine running state, errors occurred in the computational result for the oxygen storage amount due for example to variation of catalyst temperature. As a result, there was a risk that the accuracy of air-fuel ratio control would increase and exhaust emissions would worsen. Further, when the catalyst deteriorates, due to the maximum oxygen storage amount decreases, so there was a risk that the target amount would relatively shift from the appropriate value, conversion efficiency of the catalyst would be down, therefore exhaust performance with time would decrease.
It is therefore an object of this invention to resolve the above problem, and provide an engine exhaust purification device wherein the high conversion efficiency of a catalyst is maintained.
In order to achieve above object, this invention provides an engine purification device comprises a catalyst provided in an engine exhaust passage, a sensor which detects an exhaust characteristic flowing into the catalyst, and a microprocessor programmed to set a oxygen storage/release rate of the catalyst according to an engine running state, to compute an oxygen storage amount of the catalyst using the detected exhaust characteristic and the oxygen storage/release rate, to compute a target air-fuel ratio of the engine so that the oxygen storage amount of the catalyst is a predetermined target value based on the computed oxygen storage amount.